Earth-boring tools for forming wellbores in subterranean earth formations may include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit.
The cutting elements used in such earth-boring tools often include polycrystalline diamond compact cutters (often referred to as “PDCs”), which are cutting elements that include a polycrystalline diamond (PCD) material. Such polycrystalline diamond cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals under conditions of high temperature and high pressure in the presence of a catalyst (such as, for example, cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high temperature/high pressure (or “HTHP”) processes.
During drilling, fixed-cutter drill bits sometimes momentarily stick at the bottom of the wellbore, which results in rapidly increasing torque on the bit. Once the torque on the bit reaches a threshold level, the bit will slip back into rotation resulting in a decrease in the torque on the bit. The bit can oscillate between such sticking and slipping at a relatively high frequency, and such oscillation may be manifested in the form of vibrations in the drill string. This phenomenon is known in the art as “stick-slip.” FIG. 1 is a graph of RPM and depth-of-cut (DOC) (expressed in terms of inches advanced into the formation per revolution of the drill string) of a drill string at both the drill bit and at the surface of the formation, as a function of time over a five second interval. As shown in FIG. 1, in the “stick” phase (shown generally at reference numeral 10), the DOC of the drill bit increases and the RPM decreases. In the “slip” phase (shown generally at reference numeral 20), the RPM of the drill bit increases, and the DOC decreases. The RPM and DOC at the surface may remain substantially stable while the drill bit is experiencing stick-slip, also as shown in FIG. 1.
Stick-slip vibrations of drill strings have been studied by researchers for several decades. The subject is gaining renewed interest as operating parameters for PDC bits have shifted to the stick-slip regime of higher bit weight and lower rotary speed for enhanced drilling performance. Stick-slip has been identified in the art as a primary cause of bit damage. Various theories for mitigating stick-slip have been set forth in the art. Although the phenomenological basis of these theories has been provided, validation in most cases is based on anecdotal evidence from the field. Data with diagnosis based on down-hole measurements in a controlled environment has been relatively limited. Consequently, conflicting opinions continue to exist about the validity of the various theories set forth in the art for mitigation of stick-slip.
Drilling vibrations have been actively pursued by researchers for a long time as they can result in the failure of bits and BHA components and lead to increased drilling costs due to non-productive time (NPT) and reduced efficiency. For the past two decades, much of the attention in the art to reduction of drill string vibrations has been given to combating backward whirl through anti-whirl bit designs. Meanwhile, cutter technology has progressed dramatically with much more impact and abrasion-resistant, thermally stable PDC cutters. Consequently, the operating parameters for PDC bits have gradually shifted to higher weight on bit (WOB) and lower rotary speed for enhanced drilling performance.
As shown in FIG. 2, low WOB and high RPM may result in bit whirl, while higher WOB and lower RPM may result in torsional instability that matures into stick-slip vibrations. Relatively stable drilling is often encountered between these two regimes.
In view of the above, mitigation of stick-slip vibrations is gaining a renewed interest in the art.